Voltage conversion utilizing a high step-up ratio, such as on the order of 20:1 or greater and having output power levels greater than one kilowatt can be accomplished by many present methods in the art. However, these methods either suffer from a relatively low efficiency, on the order of about 85%, or require relatively complex circuitry that results in a correspondingly high cost. In addition, the step-up commutation frequency for power levels above about one kilowatt does not typically exceed 50 kHz due to component limitations. When faced with the task of designing a relatively low-cost, high-efficiency, isolated step-up DC-DC converter with demanding specifications, such as an output power of about 2 kilowatts with an input voltage of about 8-16 volts DC and an output voltage of about 400 volts DC with voltage and current control to be done over a range of no-load to full-load, present voltage conversion methods become even less feasible.
In particular, when the commutation current is relatively high, such as on the order of 250 amperes, it is difficult to suppress current and/or voltage spikes, driving a need to decrease the commutation frequency. This results in an increase in the physical size of associated magnetic components and, consequently, increased cost. This shortcoming can be partially mitigated with zero current switching (“ZCS”) available in the art for use in high-current circuits, as well as using variable frequency regulation. However, ZCS introduces another problem because when the input voltage is at a maximum the current stress on switching semiconductors and the peak flux density in step-up transformers are both increased. This, in turn, drives a need for an increased number of semiconductors and a physically larger step-up transformer to achieve acceptable converter reliability. This results in a decrease in voltage conversion efficiency and a corresponding increase in the cost of voltage conversion. Moreover, voltage conversion techniques presently available in the art are prone to significant electromagnetic interference (“EMI”) emissions, making such techniques incompatible with many applications that are sensitive to EMI, such as radio frequency (“RF”) receiving equipment.
There is a need for a physically small voltage step-up converter that is both highly efficient and cost effective as compared to techniques available in the present art, and which does not generate substantial EMI emissions.